Heavy Fraction Organic Matter Research Articles

Organic matter degradation and bacterial communities in surface sediment influenced by Procambarus clarkia.

To alleviate excessive organic matter (OM) accumulation in sediments and reduce the risk of endogenous water pollution and eutrophication in aquaculture ponds, an 84-day experiment investigated the effect of the red swamp crayfish Procambarus clarkii on the OM degradation and bacterial communities in sediments. The experiment established two groups, P. clarkia treatment and control (represented as PG and CG, respectively), with three replicates for each group. At the end of experiment, the total, light fraction, and heavy fraction organic matter concentrations in the sediment of the PG group were significantly lower than those of the CG group. Significantly higher oxidation-reduction potential (ORP) and more extensively degraded OM, indicated by fatty acids, were observed in the PG group. Compared to the CG group, the average OM removal efficiency induced by crayfish in the PG group was 15.24%. Using 16S ribosomal RNA (rRNA) high-throughput sequencing, we investigated the differences in benthic bacterial communities between the PG and CG groups. Linear discriminant analysis (LDA) effect size (LEfSe) analysis revealed that Nitrospirae, Nitrospira, Alphaproteobacteria, OLB14, Nitrospirales, Rhodobacterales, Rhizobiales, Micrococcales, Nitrospiraceae, Rhodobacteraceae, Nitrospira, Rhodobacter, Thermomonas, and Denitratisoma were significantly enriched in the PG group. Four significantly different functional groups related to OM degradation were determined between the PG and CG groups according to the functional annotation of procaryotic taxa (FAPROTAX) analysis. These four functional groups, aerobic chemoheterotrophy, manganese oxidation, dark iron oxidation, and dark sulfide oxidation, showed significantly higher relative abundances in the PG group. Overall, P. clarkia effectively increased the ORP values of sediments to provide favorable conditions for OM degradation and changed the composition and function of bacterial communities to improve bacterial abilities for OM decomposition, thereby promoting OM degradation in the sediment.

Molecular composition and structure of organic matter in density fractions of soils amended with corn straw for five years

Corn straw is an important source of carbon (C), and when applied to soil, it alters the accumulation and distribution of organic C. However, the mechanistic pathways by which newly added C is stored and stabilized in soil remain a subject of interest and debate among scholars. In this study, we investigated the chemistry of organic matter in different soil density fractions of Haplic Cambisol (sandy clay loam) separated from no-till (NTS), minimum tilled (MTS), and conventional tilled (CTS) fields amended with 8900 kg ha−1 year−1 of corn straw. After five years of corn (Zea mays L.) monocropping, soils were collected from the 0–20 and 20–40 cm depths, and processed to obtain their light fraction, occluded particulate, and heavy fraction organic matter(LFOM, oPOM, and HFOM, respectively). The results showed that compared with CT (conventional tillage without corn straw return), corn straw-amended soils (i.e., NTS, MTS, and CTS) increased soil organic C content by 11.55–16.58%. Thermogravimetric and Fourier transform infrared analyses demonstrated that the HFOM was characterized by a greater proportion of easily biodegradable substances, which may be due to the deposition of microbially processed materials on the surface of soil minerals. The LFOM and oPOM fractions were distinguished by their greater phenolic, aromatic C, and thermally stable compounds. Compared with CT, the NTS and MTS fields showed higher abundances of hydrophobic, aliphatic, and thermally unstable organic compounds, which increased soil C content and stability in the HFOM. Therefore, NTS and MTS may be ideal practices to increase soil organic C content in Haplic Cambisol.

Latitudinal patterns of light and heavy organic matter fractions in arid and semi-arid soils

Semi-arid and arid ecosystems are important for the global C cycle. Despite this, it remains unclear how organic matter fractions vary across latitudinal gradients, and what drives this variation, in dry ecosystems. In this study, we sampled soils from 100 sites across a latitudinal gradient in the dry valleys of southwestern China to explore the latitudinal patterns of light fraction organic matter (LFOM) and heavy fraction organic matter (HFOM) at two soil depths (0–10 cm and 10–20 cm). Across the studied gradient, HFOM accounted for a larger fraction of soil organic matter than LFOM. LFOM increased exponentially with increasing latitude at both 0–10 cm and 10–20 cm depths. Heavy fraction organic C increased linearly with increasing latitude at both depths, while heavy fraction organic N only increased with latitude in soils from 10 to 20 cm depth. Latitudinal patterns of LFOM were mainly explained by climate, with the most important driver being mean annual temperature, followed by mean annual precipitation. Soil physicochemical factors – in particular cation exchange capacity and silt content – explained the most variation in HFOM. Total microbial biomass was also important in explaining variation in HFOM, especially in the 10–20 cm soil layer. Overall, our results shed light on the spatial distribution of organic matter fractions in arid and semi-arid regions. We also identify candidate drivers of the variation in LFOM and HFOM in arid and semi-arid regions, finding that climate primarily explains variation in LFOM while soil physiochemistry primarily explains variation in HFOM.

No-Tillage Improvement of Nitrogen Absorption and Utilization in a Chinese Mollisol Using 15N-Tracing Method

To better understand the mechanism of nitrogen (N) distribution, absorption, utilization and loss in fertilizer under different tillage practices, a study was conducted to quantitatively explore the fate of fertilizer N in the soil–plant–atmosphere using the 15N labelling technique under the long-term conservation tillage experiment in Northeast China. The test crop used was corn. This study compared the residual amount of 15N fertilizer in soil, the content of 15N fertilizer N in particle organic nitrogen (PON), light fraction organic matter nitrogen (LFOMN) and heavy fraction organic matter N (HFOMN) under different tillage practices. In addition, N uptake, utilization and distribution by corn, the emission of N2O and the gas loss of fertilizer N, and the fertilizer N utilization rate were also taken into account. The results showed that no tillage (NT) had a significantly lower amount of residual 15N fertilizer than a moldboard plow (MP) (p < 0.05). In general, the content under NT at the 0–30 cm soil layer was 7.85% lower than that of MP. NT led to significantly greater PON and LFOMN of soil organic N compared to MP (p < 0.05). 15N from N uptake, fertilizer absorption and utilization under NT were significantly higher than that under MP (p < 0.05), the soil N absorbed by plants under NT or MP was greater than 70%. The distribution of 15N from N fertilizer in each corn part increases in this order: seed > leave > sheath > stem > bract > ear; about 57.91–64.92% of 15N is distributed in the grain. NT resulted in significantly lower average and cumulative N2O emissions than those from MP based on the static closed chamber approach (p < 0.05). The average and cumulative emissions of soil fertilizer 15N-N2O under MP were also significantly greater than that of NT. Among the N2O emissions, 15.3% and 22.98% came from fertilizer N under NT and MP, respectively. On average, 0.1–0.16% of fertilizer N was lost in the form of N2O. There was a significant difference in fertilizer utilization between NT and MP, and NT was 4.23% larger than MP (p < 0.05). These one year findings suggest that NT plays a positive role in improving the N absorption and utilization of fertilizer in a Chinese mollisol and long-term effects need to be further studied.

Long‐term fertilization effects on soil organic carbon stocks in the irrigated desert soil of NW China

AbstractSoil samples from the Hexi Corridor located at the arid regions of NW China were collected from a 23‐y‐old field experiment. Effects of fertilization on soil organic C (SOC) fractions and the distribution of added OC in the different SOC fractions were studied. The results showed that the long‐term applications of sole organic manures or in combination with inorganic N fertilizer significantly (P < 0.05) increased the OC stocks in the particulate organic matter (POM), light fraction organic matter (LFOM), and heavy fraction organic matter (HFOM) by 24.8–159.6%, 55.2–224.4%, and 6.2–57.8%, respectively, compared to the non‐fertilized treatment (CK). The average OC stock in the HFOM was higher than those of the POM and LFOM, and the conversion rate of the added OC to the OC stock of the HFOM was also higher than those of the POM and the LFOM, showing that SOC was mainly sequestrated in the HFOM. The increase of OC stock in the HFOM resulted in the increase of OC stocks in the loosely bound humus, stably bound humus, and tightly bound humus. The OC stocks in the loosely, stably, and tightly bound humus accounted averagely for 16, 19, and 65% of the OC stock in the HFOM. Furthermore, the conversion rates of the added OC to the OC stocks of loosely, stably and tightly bound humus were 2.3%, 1.5%, and 5.5%, respectively. Additionally, a significant (P < 0.01) positive linear correlation was found between the annually added OC amounts and the C sequestration rate. These results indicated that tightly bound humus was the most important component for OC sequestration in the irrigated desert soils of the Hexi Corridor and that SOC is not saturated in this region.

Soil organic matter in density fractions as related to vegetation changes along an altitude gradient in the Wuyi Mountains, southeastern China

Density fractionation can be a useful means of separating soil organic matter (SOM) into light and heavy fractions. Our objective was to evaluate SOM storage in density fractions and the relative contribution of the light fraction to total SOM pool under four different vegetations, including evergreen broad-leaved forest (EBF), coniferous forest (CF), dwarf forest (DF) and alpine meadow (AM), along an altitude gradation in the Wuyi Mountains of southeastern China, and to investigate the factors controlling the pool size in density fractions. Here we separated light and heavy fractions using 1.7 g cm −3 sodium iodide solution. The concentrations of C and N in light and heavy fractions were the highest in AM soils, while the C/N ratios in density fractions were the highest in CF soils across soil depths. The amounts of C and N in the heavy fractions increased with increasing silt plus clay content in the corresponding soil depth. Greater SOM storages in light and heavy fractions under AM suggested that the accumulation of SOM at higher altitude (lower soil temperature and higher soil moisture) mainly depends on slower decomposition rate despite lower organic matter inputs rate. The relatively larger contribution of the light fraction to total SOM under CF than that under the other three vegetation types is mainly attributed to its higher amount and quality of litterfall input and greater fine root biomass. Significant correlation existed between light fraction C and N, dissolved organic C, microbial biomass C and N, indicating considerable cycling of C and N between these pools. A stronger correlation between heavy fraction C and microbial biomass C suggested that the heavy fraction organic matter could be associated with most microbial biomass.

Organic Matter Composition in Sediments of the Baiyangdian Lake in China

Abstract The content and composition of organic matter (OM) in four areas(contrast area, aquaculture area, aquatic area and sewage area)of Baiyangdian lake are studied by using physical chemistry methods. The main objective of the study is to provide theoretical data for the management of Baiyangdian lake eutrophication. Compared with contrast area, aquaculture area and aquatic area sediment shows significantly higher concentrations of total organic matter in sewage area. These results suggest that more serious organic pollution in the sewage area sediment. Irrespective of the higher content of total organic matter, the light fraction organic matter (LFOM) is very low in four sample stations. Heavy fraction organic matter (HFOM) is the dominant class among organic matter composition in contrast area aquaculture area aquatic area and sewage area sediments contributing 85.80%, 87.7%, 78.71% and 85.00% respectively. These results indicate that HFOM account for most of organic matter while LFOM was completely decomposed. Correlation analysis of organic matter shows that labile organic matter (LOM), dissolved organic matter (DOM), LFOM and HFOM have remarkable positive correlation between them respectively.

Fungal response from oat (Avena sativa) plants and surface residue in relation to soil aggregation and organic carbon

The influence of soil fungi on soil organic carbon (OC) from surface residue was tested in outdoor plots in southern Ontario, Canada, 2004. Fungal hyphal length, soil aggregation, OC and light and heavy fractions of organic matter were compared with factors of plant growth (with or without oat [Avena sativa]) and surface residue (no residue, oat straw (low C:N) or corn (Zea mays) stalks (high C:N)) in a factorial arrangement. Significant increases were observed in soil OC from the oat plants, and from corn stalks compared to straw residue, in the growing season with very moist, high OC, sandy soil. In treatments with corn stalk residue, fungal hyphal length was increased with interaction from the oat plants and residue and was positively correlated with the heavy fraction organic matter along with soil OC. Fungal hyphae, plant roots and high C:N residue were all factors in soil OC increases.

Soil C and N cycling in three semiarid vegetation types: Response to an in situ pulse of plant detritus

Plant detritus is an important source of labile C that drives soil microbial growth and regulates the balance of N mineralization and immobilization. In semiarid ecosystems, timing of plant detrital inputs may be especially important in regulating microbial C and N cycling because of the relatively short window of time when moisture is available. Low soil moisture in early-summer may inhibit microbial colonization of recently released detritus, resulting in C-limitations to microbial growth, and this may explain the NO 3 − accumulation commonly observed in semiarid, arid, and Mediterranean ecosystems. We examined linkages between soil C availability and gross N cycling rates during summer in three common semiarid vegetation types: sagebrush, crested wheatgrass, and cheatgrass. To determine whether dry soils inhibit microbial colonization of plant detrital inputs, we stimulated soil C availability in situ by killing plant biomass shortly before the summer dry-season with herbicide (detrital-pulse treatment). Soil C and gross N cycling rates were determined during field incubations of intact soil cores from untreated soils on three occasions from late-spring to late summer, and from detrital-pulse treated soils on two occasions in summer. We hypothesized that greater C availability, resulting in increased microbial biomass and C mineralization rates, would translate to greater N immobilization rates, and this would inhibit the accumulation of inorganic N during summer months. There were few differences in soil C and N cycling among vegetation types. In all vegetation types, the in situ detrital-pulse stimulated soil C mineralization and gross N cycling rates compared to untreated soils; however, this treatment did not inhibit the summertime accumulation of NO 3 −. Instead, elevated N cycling rates and large labile N pools in detrital-pulse soils persisted throughout the summer. Our results combined with a model of microbial C–N dynamics indicate that microbes in detrital-pulse soils were utilizing substrates with C:N ratios 27% lower than in untreated soils ( p < 0.04), and much lower than expected based on the C:N of plant detritus. This suggests that substrates released by senescing plants had much lower C:N than would be predicted based on the overall C:N of plant tissue. In addition, appearance of 15N in different soil density fractions showed that the detrital-pulse treatment stimulated microbial N immobilization in both C-rich and N-rich soil microsites. Greater N immobilization associated with light fraction organic matter is consistent with greater microbial growth due to earlier input of plant detritus. Interestingly, heavy fraction organic matter was also an important sink for immobilized N and was strongly stimulated by the detrital-pulse treatment, indicating that this fraction is not as recalcitrant as formerly thought.

Dynamic of organic matter in the heavy fraction after abandonment of cultivated wetlands

The abandonment of cultivated wetland soil increased the contents of light fraction organic matter (LFOM), heavy fraction organic matter (HFOM) and soil organic matter (SOM). The LFOM and HFOM content increased to 13.3 g kg−1 and 62.4 g kg−1 after 5 years whereas they were 8.4 and 47.9 g kg−1 after 9 years of cropping, respectively. Fourteen years after abandonment, HFOM content increased to 104.3 g kg−1. LFOM was positively correlated with HFOM (p < 0.001). A Langmuir equation was used to calculate the highest HFOM value. The value for the natural wetland soil was closed to this theoretical value (140.8 g kg−1). After 14 years of abandonment, the HFOM maximum (HFOMMax) value was lower than the equilibrium value suggesting that a further increase in HFOM can occur after abandonment. Assuming a linear accumulation (3.87 Mg C ha−1yr−1), it would take approximately 24 years after the abandonment to reach the HFOMMax value.

LONG-TERM IMPACTS OF AGRICULTURE ON SOIL CARBON AND NITROGEN IN NEW ENGLAND FORESTS

Abandonment and reforestation of agricultural lands has been a major influence on the landscape of eastern North America. Cultivation and soil amendments can dramatically alter soil nutrient pools and cycling, yet few studies have examined the long-term (>50 yr) influence of pasturing and cultivation on soil processes in the forests that develop after abandonment. Twelve forested sites at Harvard Forest in central New England were compared 90–120 yr after abandonment from agricultural use. We measured soil carbon (C), nitrogen (N), and phosphorus (P); light fraction C, N, and δ15N; microbial chloroform-N; net N mineralization and nitrification; nitrification potential; and culturable nitrifiers on sites with differing land-use history and vegetation. The sites had similar soil series and topography but were arrayed along a soil disturbance gradient from permanent woodlots (selective logging but no mineral soil disturbance) to formerly pastured sites (clearcut and grazed but no deep [>10 cm] soil disturbance) to formerly cultivated sites (cleared-with-plow horizon 15–20 cm thick). Mineral soil C (0–15 cm soil depth) was very similar among all sites, but the forest floor C was lower in the cultivated sites than in the woodlots of both stand types. Mineral soil in cultivated sites contained 800 kg N/ha and 300 kg P/ha more than woodlots, a relative increase of 39% for N and 52% for P. The cultivated soils had lower C:N and C:P ratios, largely driven by higher soil N and P. The light fraction appeared to be more sensitive to prior land use than the bulk soil organic matter. The C content and C:N ratio of light fraction were lower in cultivated soils, which suggests that input and/or turnover of organic matter pools of relatively recent origin remain altered for a century after abandonment. Similar δ15N for the light and heavy fraction organic matter pools in cultivated soils suggests that cultivation accelerates the mineralization of humus N, increasing the proportion of N available for plant uptake, resulting in a convergence of the light and heavy fractions. The N pool in the woodlot soils may have been subject to preferential losses of small amounts of 14N over a longer time period, resulting in a more pronounced divergence between the light fraction (reflecting recent plant inputs) and the mineral-associated heavy fraction (more recalcitrant). Nitrification was strongly influenced by land-use history, with highest rates in formerly cultivated sites. In contrast, soil net N mineralization and chloroform-N were more strongly influenced by present vegetation. Nitrifying bacteria were relatively abundant in all pastured and cultivated sites; however, only the cultivated hardwood sites, with lowest C:N ratios (16–18), had substantial net nitrification. Historical manure inputs may explain the more rapid soil net nitrification rates, by decreasing soil C:N ratios and thus reducing nitrate immobilization in the mineral soil of cultivated sites. Regionally, 65% of the land area was pastured, and a proportion of the nutrients obtained from grazing was transferred to the cultivated croplands, which comprise ≤15% of the land area. Pastures generally had intermediate nutrient ratios and N transformations but were often more similar to woodlots, which suggests that plowing and amendments, rather than forest clearance, have the greatest impact on soil organic matter and nutrients. The influence of land-use history on soil N and P and nitrification rates was more dramatic in hardwood sites, which indicates that characteristics of the recovering vegetation and/or changes in plant community composition associated with prior land use are important factors in the rate of recovery. Our findings lead to the surprising conclusion that 19th century agricultural practices decreased forest floor nutrient content and ratios, and increased nitrifier populations and net nitrate production for approximately a century after abandonment. Consideration of site history clearly deserves more attention in the design of field experiments, and in our understanding of patterns of element distributions and transformations in dynamic forested landscapes.

Light-fraction soil organic matter: origin and contribution to net nitrogen mineralization

Two investigations were conducted to examine the nature of light-fraction (< 1.75 Mg m −3) soil organic matter (OM): (1) determination of the relative contributions of above- and belowground litter to the light fraction (LF) pool in two oak stands (Noe Woods and Wingra Woods) that are part of a long-term (>30yr) litter manipulation study in southern Wisconsin, U.S.A.; and (2) evaluation of the monthly variation in LF mass and LF N mineralization potential, plus the contribution of LF to total net N mineralization, for a cornfield, pine stand, and maple stand in western Massachusetts, U.S.A. The long-term treatments at the Wisconsin plots include no-litter, 2 × litter, and one-time removal of the A horizon. In the first study, LF mass under the no-litter treatment was 64 and 32% lower for the Noe and Wingra sites, respectively, relative to the controls (unaltered litter inputs); LF mass under the 2 × litter treatment was 85% higher at Noe, but 15% lower at the Wingra site. The LF represented only 6.7 and 4.3% of the total mineral-soil C losses under no-litter and 7.0% (Noe alone) of the total mineral-soil C gain under 2 × litter. The results suggest that aboveground foliar litter contributed one-third (Wingra) to two-thirds (Noe) of the LF pool. Additionally, the heavy fraction (HF) declined quickly in response to relatively short-term changes in litter input and functioned as the primary long-term C sink. In the second study, LF represented 11% (corn), 13% (pine), and 2% (maple) of the N mineralization potential (anaerobic incubation) for the whole mineral soil; contribution of LF to net N mineralization could not be determined for the cornfield, but was calculated to be 3 kg N ha −1 yr −1 for both the pine and maple stands. N mineralization potential for the pine and maple sites was consistently lower for LF OM than for HF OM. Both LF mass (corn and pine) and LF N mineralization potential (all sites) showed differences by sampling month. LF constituted 5% (maple), 13% (corn), and 14% (pine) of mineral soil OM. Findings suggest that HF is the primary N source in coarse-textured mineral soils, and also that LF is a relatively minor N source in forest stands with mor-type soil.

Processes of Soil Organic‐Matter Accretion at a Mudfloe Chronosequence, Mt. Shasta, California

Amounts of carbon and nitrogen were measured in bulk soils and compenent density fractions from four mudflows (A, B, D, E) of increasing age at Mt. Shasta, California, in order to gain insight into mechanisms of soil organic—matter accretion. Soil was divided into a light fraction (specific gravity &lt; 1.65 g/cm3) in which organic matter consisted mainly of root fragments and other plant remnants and a heavy fraction in which most of the organic matter was adsorbed on mineral surfaces and occluded within organomineral microaggregates. In bulk soil and the heavy fraction, amounts of C and N increased consistently with flow age and were even greater at an older, mineralogically similar site previously studied in the Oregon Cascade Range. Amounts of light—fraction C and N increased from flow A to D but were lower at flow E. Regardless of soil age, the heavy fraction contained 55—88% of the total soil N and 37—72% of the total soil C. Thus, the heavy fraction is the major long—term sink for N and an important sink for C. Soil properties and vegetation were exceptionally variable at the D and E flows. A multiple regression on rockiness of the soil (volume of fragments &gt; 2mm) and a tree index (reflecting number and size of trees near each soil—sampling point and their proximity) accounted for 50—80% of the variability in light—fraction C and N at the D and E flows but not at the A and B flows, where all parameters were less variable. Rockiness and tree index accounted for little of the variability in heavy—fraction C and N, indicating that light— and heavy—fraction organic—matter accumulate in response to different factors. It appears that mass and composition of light—fraction organic matter correlate with short—term changes in biological activity, whereas heavy—fraction organic matter accumulates steadily and correlates with long—term processes such as mineral weathering and soil horizon formation.